1. Field of the Invention
This invention relates generally to a system for recording and/or reproducing digital data and, more particularly, to a system wherein a floppy disk, as employed for use in an electronic still camera, is used to store digital data for other purposes, for example, for a computer.
2. Description of the Background
The presently well-known and widely used 8-inch, or 5.25-inch, magnetic, floppy disk has a standardized format and almost all of the presently known disk drive systems operate under such standardized format. As a result, it is very difficult to employ the most up to date and emerging technologies to realize higher density recording and/or reproduction with the standardized systems. Furthermore, because the rotation speed of a magnetic disk employed in the known floppy disk system is usually either 300 rpm or 600 rpm, it is essentially impossible to record and/or reproduce an analog video signal in real time. If the video signal is digitized it can be recorded and/or reproduced in real time, however, one floppy disk is capable of at best recording the information of only one still video frame on a standard 8-inch or 5.25-inch disk. In addition to requiring an analog-to-digital (A/D) converter and a complimentary digital-to-analog (D/A) converter to record and reproduce the video information digitally a digital video system also requires a frame memory. Therefore, the total digital floppy disk system becomes expensive and physically large and unwieldy. Accordingly, it is not practical to use the presently known floppy disk system to record and/or reproduce a video signal.
An electronic still camera conference in Japan has proposed the use of a 2-inch floppy disk as the recording medium for all electronic still cameras. Such 2-inch floppy disk generally resembles the presently known floppy discs but is 47 mm in diameter, 40 .mu.m in thickness, and it has a center core element by which is may be engaged with a suitable drive mechanism. A rotational position detection element is also affixed to the center core. The 2-inch floppy disk resides within a jacket having a central opening to expose the center core element and a slideable opening so that the disk may be brought into operative contact with a magnetic head. When the disk is not in use, the slideable opening has a cover or shutter which is dust-proof and seals the disk from adverse environmental effects. A counter dial is provided to indicate the number of exposures on the disk, and a tab is provided that may be broken out to prevent inadvertent re-recording.
The data format of the proposed 2-inch disk has been preliminarily agreed upon as comprising fifty tracks, with each track with being 60 .mu.m in width, having a guard band between adjacent tracks of 40 .mu.m. The disk is proposed to be rotated at 3600 rpm, which represents the field frequency, and a video color signal of one field is to be recorded in each track of the magnetic disk. Thus, the proposed standardized floppy disk can be used to record fifty still color video signals. Nevertheless, although such 2-inch diameter floppy disk has been standardized to record and/or reproduce analog color video signals, it cannot easily accommodate other digital data. For example, if some other kind of digital data is converted to a quasi-video signal and then recorded on the floppy disk, such as might be done by an audio pulse code modulation (PCM) processor employed in a video tape recorder (VTR), the floppy disk will have a small memory capacity relative to the original digital data. Moreover, other problems arise, such as data compatibility, in using such disks with the existing 8-inch or 5.25-inch floppy disk systems.
When the digital data is recorded on or reproduced from this 2-inch still video camera floppy disk, it is done so in keeping with the format generally known for use in other floppy disk systems. Accordingly, when the 2-inch floppy disk is viewed from the standpoint of the video signal, it appears as having a very high recording density, whereas when viewed from the standpoint of the digital data there is a low recording density and, thus, the floppy disk is not being fully utilized.
If the video signal and the digital data were to be recorded and/or reproduced from a single floppy disk in an intermixed condition, because the signals are largely located in different bands and have different characteristics, it becomes difficult to record and/or reproduce such intermixed video signals and digital data with optimum conditions. This is due in part to the electromagnetic transducer characteristics involved, head-disc contact conditions, and so on. Furthermore, when the video signal and the digital data are recorded and/or reproduced in an intermixed state, the drive unit employed for rotating the floppy disk must rotate the disk at 300 rpm (or 600 rpm), in the case of digital data, and must also rotate the disk at 3600 rpm, in the case of the video signal. Thus, when the revolution speed of the floppy disk is selectively changed other problems arise because the floppy disk is unavailable for access for several seconds, while the servo is being stabilized. Also, requiring two motor speeds further increases manufacturing costs.
One format that could be used to record and/or reproduce video information and digital data on a floppy disk would divide the tracks on the disk into four intervals called BLOCKS, each having approximately a 90.degree. sector. Each block could then contain an index portion and a series of data frames that would contain both the information to be recorded and some manner of error correcting code, such as a parity code or the like. Also, it is required that the digital sum variation (DSV) must be small, the ratio between the minimum length between transitions (T.sub.min) and the maximum length between transitions (T.sub.max) must be small, and the window margin must be made large. In order to accomplish this then an up-code conversion is required, such as the eight-to-ten conversion with T.sub.max =4T. In this way, by following such specific encoding and data organization a 2-inch floppy disk can be made to have a large capacity, in spite of its relatively small size.
In the use of presently known floppy disks, the data transfer between the disk and the peripheral devices employed in the system is typically carried out directly at a speed that is determined by the speed of rotation of the disk, without requiring the use of a buffer memory between the disk output and the peripheral device. Also, data is allocated on the the floppy disk so that data is written in or read out from the disk at a sector unit such that the address data is consecutive. That is, the time sequence of data recorded on the floppy disk is continuous relative to the original time sequence. Nevertheless, when employing a floppy disk as proposed hereinabove, a higher transfer speed is required because the digital magnetic recording density is higher. Also, because redundant bits are added to the data for error correction and because the data is frequently rearranged by interleaving in order to correct burst errors, the time sequence of the resultant data is not in correspondence with the original time sequence on the disk. Accordingly, the floppy disk cannot be connected with the peripheral device due to this lack of consecutive time sequence. Thus, it is necessary to interpose a buffer memory between the disk and the peripheral device, however, if the data allocation on the recorded pattern of the disk is written or read out from the buffer memory as it is, the logical data addresses as viewed from the peripheral device will still not be seen to be continuous. Accordingly, when data is to be transferred in a direct memory access (DMA) mode in order to transfer data at high speed, because the data must be transferred with its addresses being consecutive, the addresses will not be matched with the buffer memory and high speed data transfer cannot be obtained.
One approach to overcoming this above-described short-coming would be that upon recording after the addition of the parity data, and during reproduction after error correction the data within the buffer memory would be rearranged, so that data which is stored in the buffer memory has its addresses consecutively arranged in the original time sequence. Nevertheless, this involves substantial additional memory capacity, as well as the requirement for the conversion time necessary to rearrange the data, all of which are not desirable.